Digital signal recording/reproducing method

ABSTRACT

Digital signal sequences to be recorded in predetermined unitary recording areas are transformed into blocks, a plurality of bit data carrying all digit levels to be read and determined upon reproduction are included in the resultant blocks, and sequences of the data blocks are recorded in the recording medium in the unitary recording areas. Upon reproduction, the blocks of the read signals obtained from a recording medium are recognized, a threshold value is determined for each block on the basis of the values of read signals corresponding to bit data carrying all digit levels to be read and determined, and each digit level of bit data in the read signal are read and determined on the basis of the threshold values.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to an informationrecording/reproducing method, and more particularly to a method ofrecording a digital signal on a recording medium and reproducing thedigital signal therefrom.

[0003] 2. Description of the Related Art

[0004] A holographic memory system, which is one of informationrecording/reproduction systems mentioned above, records a digital signal(hereinafter called “data”) on a holographic memory medium (aphoto-refractive crystal material such as LiNbO₃ or the like) andreproducing the data therefrom. The system is capable of recording andreproducing data in a form of two-dimensional plane page, and moreovercapable of recording and reproducing over a large number of pages. Anexemplary configuration of the system is illustrated in FIG. 1.

[0005] Referring to FIG. 1, an encoder 11 converts time series recordingdata sequences to be recorded into a “page” in a holographic memorymedium 1. In other words, the encoder 11 rearranges the time seriesrecording data into a data matrix corresponding to a two-dimensionalunitary plane page as a predetermined unitary recording region, forexample, in a matrix of vertically 480 bits and horizontally 640 bits(480×640) to produce unitary page data sequence. The unitary page datasequence is sent to a spatial light modulator (SLM) 12.

[0006] The SLM 12 optically modulates an irradiated signal beam inaccordance with the unitary page data sequence from the encoder 11 inmodulation processing units of vertically 480 pixels×horizontally 640pixels corresponding to the unitary page, and leads a modulated beamresulting therefrom to a lens 13. More specifically, the SLM 12 passes asignal beam therethrough in response to a logical value “1” in theunitary page data sequence, which is an electrical signal, and blocksthe signal beam in response to a logical value “0,” to achieveelectrical-optical transducing in accordance with the respective bitcontents in the unitary page data to produce a modulated signal beam asan optical signal representing the unitary page sequence.

[0007] Such a modulated signal beam is input to the holographic memorymedium 1 through the lens 13. The holographic memory medium 1 is alsoirradiated with a reference beam having an angle from a predeterminedreference line (hereinafter, called the “incident angle β”) orthogonalto the optical axis of the beam carrying the optical signal, other thanthe modulated signal beam.

[0008] When the modulated signal beam and the reference beam aresimultaneously incident on the holographic memory medium 1, both beamsinterfere with each other within the holographic memory medium 1 toproduce an interference pattern which is recorded in the holographicmemory medium 1, thereby recording the data in the holographic memorymedium 1. Also, by inputting the reference beam with a differentincident angle β, data can be recorded in the holographic memory medium1 in units of predetermined three-dimensional recording regionsincluding a plurality of pages of two-dimensional data.

[0009] For reproducing recorded data from the holographic memory medium1, unlike recording, no signal beam is input to the holographic memorymedium 1, but the reference beam only is input at the same incidentangle β as that used during the recording. In this way, diffracted lightfrom an interference pattern recorded in the holographic memory medium 1is led to a lens 21.

[0010] The diffracted light reaching the lens 21 passes through the lens21, and impinges on a CCD (Charge-Coupled Device) 22 having a lightreceiving area of vertically 480 pixels×horizontally 640 pixels. Each ofthe pixels in the light receiving area of the CCD 22 corresponds to eachpixel on a recording plane in the holographic memory medium 1, so thatthe CCD 22 transduces the brightness of the incident light in each pixelarea into the magnitude of an electrical signal level, i.e., generatesan analog electrical signal indicative of a level corresponding to theluminance of incident light, and supplies the analog electrical signalto a decoder 23 as a read signal.

[0011] The decoder 23 has a function of digitizing the read signal orperforming binary determination, and recognizes a logical value “1” whenthe read signal has a level higher than a slice level serving as athreshold value, and a logical value “0” when lower than the level toproduce a digital signal which carries the values thus recognized. Inaddition, the decoder 23 performs a reverse conversion of conversionperformed in the encoder 11 to the digital signal to produce time seriesreproduced signal.

[0012] The holographic memory system thus configured is capable ofrecording and reproducing three-dimensional data including a temporalelement to and from the holographic memory medium 1 by changing anincident angle β of a reference beam at arbitrary time intervals, aswell as recording and reproducing planar two-dimensional data invertical and horizontal dimensions.

[0013] However, a variety of factors such as dust and stain on eachoptical elements, crosstalk, interference fringes and so on may causethe light intensity to spatially and temporally fluctuate, resulting influctuations due to noise in amplitude occurring in the output of theCCD, i.e., a read signal, other than a change in amplitude due to dataitself. When the output of the CCD 22 is converted to data of “1” or “0”based on a fixed slice level in the decoder 23, data read errors mayoccur owing to amplitude fluctuations.

[0014] By way of example, assume that an image carrying atwo-dimensional data matrix as illustrated in FIG. 2 is recorded in theholographic memory medium 1.

[0015] In FIG. 2, a white portion represents data “1,” while a blackportion represents data “0.” If an irregular luminance illustrated inFIG. 3 is superimposed on the pattern image carrying such data, a readsignal is produced on the basis of a pattern image as illustrated inFIG. 4. In this case, an amplitude value of a read signal in a data “1”portion is affected by “dark” or lower luminance irregularity to becomesmaller.

[0016] In FIG. 4, when a portion free from a “dark” region due to theirregular luminance indicated by an arrow A is sliced in the horizontaldirection, and is represented as a change in the output level of lightreceived by the CCD 22 corresponding to the sliced portion, i.e., achange in the level of the read signal, a waveform as shown in FIG. 5 isderived.

[0017] In FIG. 5, by determining whether the read signal is “1” or “0”based on a slice level defined by a median value between a maximum valueand a minimum value of the read signal, recorded data “1, 0, 1, 0, 1, 0,1, 0, 1, 0, 1, 0, 1, 0, 1” can be correctly reproduced as reproduceddata “1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1.”

[0018] However, if a portion including a “dark” region due to irregularluminance indicated by an arrow B is sliced in the horizontal direction,and a read signal from that portion is determined whether it is “1” or“0” with the same slice level as FIG. 5, recorded data is not correctlyreproduced because a level corresponding to the “dark” region is lowerthan the slice level as shown in FIG. 6, so that the reproduced datarepresents “1, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1,” thus resultingin a read error.

[0019] While FIG. 6 shows an example where the level of a read signalcorresponding to data “1” becomes lower due to a “darky” region of theirregular luminance to result in a read error, the level of a readsignal corresponding to data “0” may become higher due to a “light”region of irregular luminance to cause a read error, as shown in FIG. 7.

[0020] Also, as shown in FIGS. 8 and 9, data cannot be correctlyreproduced if the entire level of a read signal fluctuates with anoffset due to the irregular luminance to cause a read error.

[0021] In the case shown in FIGS. 8, 9, while a large offset may resultin the level of a read signal exceeding an upper limit of data “1” orfalling below a lower limit of data “0,” the level is limited by therespective upper limit or the lower limit.

OBJECT AND SUMMARY OF THE INVENTION

[0022] The present invention has been made in view of the problemmentioned above, and its object is to provide a digital signal recordingmethod, reproducing method and recording/reproducing method which arecapable of reliably reproducing a recorded digital signal even if a readsignal is experiencing level level fluctuations due to noise.

[0023] To achieve the above object, a method of recording a plurality ofdigital signal sequences at predetermined unitary recording areas in arecording medium, respectively, according to the present inventioncomprises the steps of transforming each of the digital signal sequencesto be recorded into blocks, including a plurality of bit data carryingall digit levels to be read and determined upon reproduction within eachof the resultant data blocks, and recording a sequence of the datablocks at the corresponding ones of the predetermined unitary recordingareas in the recording medium.

[0024] A method of reproducing digital signal sequences from a recordingmedium according to the present invention comprises the steps ofrecognizing blocks in each of read signal sequences obtained from therecording medium, determining a threshold value for each block on thebasis of read values of bit data carrying all digit levels to be readand determined, and reading and determining a digit level of each bit ofthe read signal sequences on the basis of the threshold value.

[0025] Another method of recording a plurality of digital signalsequences at predetermined unitary recording areas in a recordingmedium, respectively, according to the present invention comprises thesteps of transforming each of the digital signal sequences to berecorded into blocks, locating reference bit data carrying at leastfirst and second digit levels to be read and determined uponreproduction at respective predetermined positions in each of theresultant data blocks, and recording a sequence of the data blocks atthe corresponding ones of the predetermined unitary recording areas inthe recording medium.

[0026] Another method of reproducing digital signal sequences from arecording medium according to the present invention comprises the stepsof recognizing blocks in each of read signal sequences obtained from therecording medium, determining a threshold value for each of the blockson the basis of read values of reference bit data carrying at leastfirst and second digit levels to be read and determined, the referencebit data being located at predetermined positions within each of theblock, and reading and determining a digit level of each bit of the readsignal sequences on the basis of the threshold value.

[0027] In one embodiment of the recording method, the data block may beformed of the number of bits corresponding to a vertical length by whicha recording plane in the recording medium is divided, and the number ofbits corresponding to a horizontal length by which the recording planein the recording medium is divided.

[0028] Alternatively, the data block may be formed of the number of bitscorresponding to a vertical length by which a recording plane in therecording medium is divided, the number of bits corresponding to ahorizontal length by which the recording plane in the recording mediumis divided, and the number of bits corresponding to a recording timeinterval for a recording area defined by both of the numbers of bits.

[0029] A further method of recording a plurality of digital signalsequences at predetermined unitary recording areas in a-recordingmedium, respectively, according to the present invention comprises thesteps of preparing correction digital signals each corresponding to eachof the predetermined unitary recording areas and each including bit datacarrying digit levels to be determined upon reproduction and having apredetermined bit position relationship corresponding to the digitlevels, mixing each of the correction digital signals with each of thedigital signal sequences, and recording the resultant digital signalsequences at the corresponding ones of the predetermined unitaryrecording areas in the recording medium.

[0030] A further A method of reproducing digital signal sequences from arecording medium according to the present invention comprises the stepsof recognizing correction digital signals for each of predeterminedunitary recording areas from the read signal sequences obtained from therecording medium, identifying bit data carrying digit levels to be readand determined from the correction digital signals on the basis of apredetermined bit position relationship thereof, detecting deviationvalues from the respective expected values of the identified bit data,and correcting values of each bit of the digital signal sequences on thebasis of the deviation values.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a block diagram illustrating the basic configuration ofan information recording/reproducing system common to a prior art andrespective embodiments of the present invention;

[0032]FIG. 2 is a schematic diagram illustrating an example of a datapattern image for a recording page;

[0033]FIG. 3 is a schematic diagram illustrating a pattern of irregularluminance which can be superimposed on the data pattern image of FIG. 2;

[0034]FIG. 4 is a schematic diagram illustrating an example of theirregular luminance pattern of FIG. 3 superimposed on the data patternimage of FIG. 2;

[0035]FIG. 5 is a time chart showing an example of level fluctuations ofa read signal in a normal operation;

[0036]FIG. 6 is a time chart showing an example of level fluctuations ofa read signal when an irregular luminance is present;

[0037]FIG. 7 is a time chart showing another example of levelfluctuations of a read signal when an irregular luminance is present;

[0038]FIG. 8 is a time chart showing an example of level fluctuations ofa read signal when an offset component is included in the read signal;

[0039]FIG. 9 is a time chart showing another example of levelfluctuations of a read signal when an offset component is included inthe read signal;

[0040]FIG. 10 is a flow chart illustrating a procedure for datareproduction processing performed in accordance with a first embodimentof the present invention;

[0041]FIG. 11 is a diagram illustrating an example of a data pattern fora unitary recording page for explaining in detail the data reproductionprocessing of FIG. 10;

[0042]FIG. 12 is a time chart for explaining data reproduced by the datareproduction processing of FIG. 10;

[0043]FIG. 13 is a diagram illustrating another example of a datapattern for a unitary recording page for explaining in detail the datareproduction processing of FIG. 10;

[0044]FIG. 14 is a flow chart illustrating a procedure for the datareproduction processing performed in accordance with a second embodimentof the present invention;

[0045]FIG. 15 is a diagram illustrating an example of a data pattern fora unitary recording page for explaining in detail the data reproductionprocessing of FIG. 14;

[0046]FIG. 16 is a time chart for explaining data reproduced by the datareproduction processing of FIG. 14;

[0047]FIG. 17 is a diagram illustrating another example of a datapattern for a unitary recording page for explaining in detail the datareproduction processing of FIG. 14;

[0048]FIG. 18 is a diagram showing a corresponding relationship ofrecording page data sequence composed by encoding processing accordingto a third embodiment of the present invention; and

[0049]FIG. 19 is a flow chart illustrating a procedure for datareproduction processing performed in accordance with the thirdembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Several embodiments of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

[0051] A general configuration of an information recording/reproducingsystem according to first embodiment of the present invention is thesame as that illustrated in FIG. 1.

[0052] However, the encoder 11 does perform processing unique to thisembodiment. Specifically, data to be recorded, corresponding to a singleunitary page or a predetermined number of unitary pages, is divided intotwo-dimensional or three-dimensional predetermined blocks such that atleast one data bit “1” and at least one data bit “0” are included ineach block. In other words, the data block includes a plurality of databits which generally carry all digit levels to be read and determinedupon reproduction. The unitary page data sequences to be recorded thuscomposed are sent to the SLM 12.

[0053] The data sent to the SLM 12 is recorded in the holographic memorymedium 1 by a signal beam and a reference beam.

[0054] The data recorded in the holographic memory medium 1 is read byusing the reference beam. In this event, diffracted light from theholographic memory medium 1 reaches the CCD 22 through the lens 21. TheCCD 22 supplies its output indicative of the received light to thedecoder 23 as a read signal.

[0055] The decoder 23 performs data reproduction processing unique tothis embodiment. A flow chart of the processing is illustrated in FIG.10.

[0056] Specifically, as the decoder 23, for example, A/D-converts theread signal to fetch digital signal data sequences (step S11), itdivides each of the digital data sequences into predetermined blocks asa block recognition step (step S12).

[0057] Next, the decoder 23 compares each level of the read data (i.e.,value of each bit) in each block with a preliminary slice levelarbitrarily determined for the block (for example, an average value ofall the read data in the block may be used as the slice level), and oncedetermines “1” or “0” for the level of the read data by checking whetherit is higher or lower than the slice level to produce a data carrying“1” or “0” (step S13).

[0058] Then, the decoder 23 calculates an average value of the read datacorresponding to data determined as “1” (at least one) within the blockproduced by the processing at Step S13, and an average value of the readdata corresponding to data determined as “0” (at least one) within theblock produced by the processing at Step S13 (step S14).

[0059] The decoder 23 newly determines a slice level from the twoaverage values so as to minimize a read error in the read data (stepS15). Then, the decoder 23 compares the read data with the newlyobtained slice level to determine “1” or “0” again for the level of theread data by checking whether it is higher or lower than the slicelevel, and newly produces determined data which carries “1” or “0” (stepS16). As a new slice level determined at Step S15, an average of the twoaverage values for “1” and “0” derived at Step S14 may be employed.

[0060] Subsequent to Step S16, the decoder 23 determines whether or notthe slice level should be changed (step S17). If it should be changed,i.e., if it is determined that the read error in data can be furtherreduced, an average value of the read data corresponding to datadetermined as “1” and an average value of the read data corresponding todata determined as “0” are again calculated for each block of thedetermined data which have been finally derived to determine the slicelevel. Then, the procedure proceeds to Step S14 to determine “1” or “0”for the level of the read data

[0061] If it is determined at Step S17 that the slice level need not bechanged, the decoder 23 combines the blocks (step S18), and outputs thefinally derived determined data as decoded data or reproduced data intime series (step S19).

[0062] The foregoing processing performed by the encoder 11 and thedecoder 23 may be implemented by a personal computer or the like eitherin software or in hardware.

[0063] As an example of two-dimensional division of recording data intoblocks, explanation will be given of a two-dimensional data matrix ofFIG. 4 which is divided into 3 bits by 3 bits blocks 1-a, 2-a, . . . ,5-e as illustrated in FIG. 11.

[0064] A block is composed to satisfy a condition that it must includeone or more data bits “1” and one or more data bits “0.” Here, one bitlocated at the upper left corner of the data in the 3×3 block is used asan adjusting bit for satisfying the condition, so that the remainingeight bits, excluding the upper left one bit, are used as normalinformation data. As a specific method of determining the value for theadjusting bit, the smaller one of the numbers of “1” and “0” ofinformation data in the block is assigned to the adjusting bit. If thenumber of “1” is the same as the number of “0” in information data of ablock, either of them, for example, “1” may be selected.

[0065] In this way, it is possible to compose a block which includesboth data “1” and “0” without fail.

[0066] According to this embodiment, when the blocked data asillustrated in FIG. 11 is read, the read data is divided into 3×3 blocksidentical to those upon encoding at Step S12. Then, at Step S13, eachbit of the read data is determined whether it is “1” or “0” with anarbitrary slice level for each block, and an average value of the readdata corresponding to “1” data thus determined and an average value ofthe read data corresponding to “0” data thus determined are againaveraged to determine a slice level at Steps S14 and S15. Then, each bitof the read data is again determined whether it is “1” or “0” with thenewly determined slice level.

[0067] Therefore, neighboring data on a recording format, here, in aunitary plane page of data to be read, i.e., data “1” and “0” within apreviously defined block are used to set a slice level to determine “1”or “0” of the data. Therefore, even if the level of read signalfluctuates due to noise caused by a variety of factors, the values ofdata within the block, which is referenced for the slice level, alsofluctuates to cause the slice level to fluctuate in a manner similar tothe data, thereby making it possible to correctly determine whether theread data is “1” or “0.”

[0068]FIG. 12 shows level fluctuations of the data corresponding to aportion of the read data, indicated by an arrow B in FIG. 11, which hasbeen sliced in the horizontal direction.

[0069] Referring specifically to FIG. 12, in blocks free from theinfluence of irregular luminance such as blocks 1-c, 2-c or the like,data can be correctly determined whether it is “1” or “0” based on aslice level determined by the average values for the read data “1” andfor the read data “0” without errors.

[0070] In blocks influenced by irregular luminance such as a block 3-c,on the other hand, the read data “1” in the block is influenced by theirregular luminance. The read data “1” and the read data “0” aredetermined with a new threshold value changed as appropriate, and theslice level determined in accordance with these read data also followthe change, so that the read data having levels fluctuating due to anoise component such as irregular luminance can also be determined for“1” or “0” without errors.

[0071] Next, a three-dimensional division of recording data into blockswill be explained with reference to an example. In this example, thetwo-dimensional data matrix of FIG. 4 is processed such that not only aplane A as a unitary recording page is divided into 3 bits by 3 bitsareas 1-a, 2-a, . . . , 5-e, but also a plane B as a unitary recordingpage produced by changing an incident angle β of a reference beam over atime T is also divided corresponding to each of divided areas of theplane A, as shown in FIG. 13, to produce a block having a thickness ofthe time T and a volume of 2 bits of plane dimension by 3 bits ofvertical dimension by 3 bits of horizontal dimension.

[0072] In such a division of recording data into blocks, it can begenerally said that a data block is formed by the number of bitscorresponding to a vertical length by which a recording plane on themedium 1 is divided; the number of bits corresponding to a horizontallength by which the recording plane is divided; and the number of bitscorresponding to a recording time interval for a recording area definedby both of the numbers of bits.

[0073] More specifically, in FIG. 13, the encoding is performed, forexample, on a virtually three-dimensional cubic block of 2 bits by 3bits by 3 bits formed by an area 1-a on a plane A and an area 1-a on aplane B, both of which are recording pages. The cubic block is the unitof the encoding.

[0074] A block is composed to satisfy a condition that it must includeone or more data bits “1” and one or more data bits “0”. Here, one bitat the upper left corner of data in a 3×3 area on the plane A is chosento be an adjustment bit, and a total of 17 bits including eight bits inthis area excluding the upper left bit and all nine bits of data in the3×3 area on the plane B are used as information data. For the adjustingbit, the smaller one of the numbers of “1” and “0” of information datain the block is assigned. If the number of “1” is the same as the numberof “0” in information data of a block, either of them, for example, “1”may be selected.

[0075] According to the embodiment, when blocked data as illustrated inFIG. 13 is read, the read data is divided into 2×3×3 blocks identical tothose upon encoding at Step S12, and determination is made as to whetherthe read data is “1” or “0” based on an arbitrary slice level for eachblock at Step S13. An average value of the read data corresponding todata determined as “1” and an average value of the read datacorresponding to data determined as “0” are further averaged at Step S14to determine the slice level at Step S15. Then, the read data is againdetermined whether it is “1” or “0” based on the newly determined slicelevel.

[0076] Thus, the “0” determination is performed by using neighboringdata on a three-dimensional recording format of data to be read, i.e.,other data within a three-dimensional block over a previously set timeinterval T to determine a slice level. Therefore, even if the level ofread signal fluctuates due to noise caused by a variety of factors, thevalues of data within the block, which is referenced for the slicelevel, also fluctuates to cause the slice level to fluctuate in a mannersimilar to the data, thereby making it possible to correctly determinewhether the read data is “1” or “0.” The approach for setting a slicelevel using three-dimensional blocks as mentioned is advantageouslyresistant to fluctuations in a recording form and a reproduction form onthe time base, and capable of performing efficient processing because ofa large amount of data contained in each block.

[0077] As described above, even if the level of the read data arrangedin two-dimensional or three-dimensional recording format spatially ortemporally fluctuates due to a variety of factors, the slice level fordetermining “1” or “0” is determined from the read data “1” and “0”which are similarly affected by the fluctuations, thereby making itpossible to determine whether each bit of the read data is “1” or “0”even if level fluctuations due to noise is present in the read data.

[0078] Moreover, fluctuations exceeding an upper limit of “1” andfalling below a lower limit of “0,” as illustrated in FIGS. 8 and 9 maycause the level of a read signal to saturate, resulting in the lack oflinearity in the level change and a significantly distorted waveform ofthe read signal. Even in such a case, the slice level is determinedutilizing information of both the read data “1” and “0” the slice levelcan be appropriately determined because even if one is saturated, theother remains unsaturated.

[0079] While the foregoing explanation has been made referring to anexample where the shape of block for determining the slice level is 3×3for a two-dimensional block, and 2×3×3 for a three-dimensional block,any shape of block may be used as long as it includes both of data “1”and “0.” Alternatively, a variety of blocks may be combined withoutlimiting the shape. Further, a block having a different shape may beprovided for determining the slice level for extracting data “1” and “0”separately from the block for determining “1” or “0” for data.

[0080] Also, while an adaptive slice level is calculated after each bitof data in block is determined whether it is “1” or “0” based on anarbitrary slice level for each block, the slice level may be calculatedby first determining the read data as to whether data is “1” or “0”based on a single slice level before it is divided into blocks, and thendividing the read data into blocks.

[0081] An information recording/reproducing system according to anotherembodiment of the present invention can be basically realized in asimilar manner to that illustrated in FIG. 1.

[0082] The encoder 11, however, does perform processing unique to thisembodiment. More specifically, recording data corresponding to a unitarypage or a predetermined number of unitary pages is divided intopredetermined two-dimensional or three-dimensional blocks, each of whichis composed to include at least one reference data “1” specified at anassigned location, and at least one reference data “0” similarlyspecified at an assigned location. In other words, the reference databits, each carrying at least first and second digit levels (“0” and “1”in this embodiment) to be read and determined upon reproduction, arearranged at the respective predetermined locations in a resulting datablock. A unitary page recording data sequence thus composed is sent tothe SLM 12.

[0083] The data sent to the SLM 12 is recorded in the holographic memorymedium 1 by a signal beam and a reference beam.

[0084] The data recorded in the holographic memory medium 1 is read byusing the reference beam. In this event, diffracted light from theholographic memory medium 1 reaches the CCD 22 through the lens 21. TheCCD 22 supplies its output indicative of received light to the decoder23 as a read signal.

[0085] The decoder 23 performs data reproduction processing unique tothis embodiment as illustrated in FIG. 14.

[0086] Specifically, as the decoder 23, for example A/D-converts a readsignal to fetch digital signal data sequences (step S21), it divideseach of the digital data sequences into predetermined blocks as a blockrecognition step (step S22).

[0087] Next, the decoder 23 extracts reference data “1” and “0” withinthe block (step S23), and determines a slice level so as to minimizeread errors in the read data from both reference data (step S24). As theslice level, an average value of the reference data “1” and “0” may beemployed.

[0088] After the slice level has been determined, the decoder 23determines “1” or “0” for the level of the read data by checking whetherit is higher or lower than the slice level (step S25). The data “1” and“0” obtained by the determination are output as decoded data, i.e.,reproduced data in time series (step S26).

[0089] The processing performed by the encoder 11 and the decoder 23 mayalso be implemented by a personal computer or the like either insoftware or in hardware.

[0090] As an example of two-dimensional division of recording data intoblocks, explanation will be given of a two-dimensional data matrix ofFIG. 4 which is divided into 3 bits by 3 bits blocks 1-a, 2-a, 5-e asillustrated in FIG. 15.

[0091] A block is composed to satisfy a condition that it must includeone or more reference data bits “1” and one or more reference data bits“0” respectively specified at particular locations in the block. Here,one bit located at the upper left corner of the data in each 3×3 blockis used as the reference data bit “1” and the next one bit to the rightas the reference data bit “0,” and the remaining portion except for thetwo locations is assigned for normal information data.

[0092] According to the embodiment, when the blocked data as illustratedin FIG. 15 is read, the read data is divided into 3×3 blocks identicalto those upon encoding at Step S22. At Step S23, the reference data “1”recorded at the upper left corner and the reference data “0” recorded atthe next location to the right of the reference data “0” are extractedin each block at Step S23. Then, at Step S24, the slice level isdetermined by the extracted reference data, and each bit of the readdata is determined whether it is “1” or “0” based on the determinedslice level.

[0093] Therefore, neighboring data on a recording format of data to beread, i.e., the reference data positioned at previously specifiedlocations in a block are used to set a slice level to determine “1” or“0” of the data. Therefore, even if the level of read signal fluctuatesdue to noise caused by a variety of factors, the values of the referencedata within the block, which is referenced for the slice level, alsofluctuates to cause the slice level to fluctuate in a manner similar tothe reference data, thereby making it possible to correctly determinewhether the read data is “1” or “0.”

[0094]FIG. 16 shows level fluctuations of the read data corresponding toa portion of the read data , indicated by an arrow B in FIG. 15, whichhas been sliced in the horizontal direction.

[0095] Referring specifically to FIG. 16, in blocks free from theinfluence of irregular luminance such as a block 1-d or the like, datacan be correctly determined that it is “1” or “0” based on a slice leveldetermined by the reference data “1” and the reference data “0” withouterrors.

[0096] In blocks influenced by irregular luminance such as a block 3-d,on the other hand, the reference data “1” in the block is influenced bythe irregular luminance in a manner similar to other data. Thus, thereference data “1” and the reference data “0” are determined with a newthreshold value changed as appropriate, and the slice level determinedin accordance with these read data also follow the change, so that theread data having level fluctuations can also be determined for “1” or“0” without errors.

[0097] Next, a three-dimensional division of recording data into blockswill be explained with reference to an example. In this example, thetwo-dimensional data matrix of FIG. 4 is processed such that planes Aand B, which are recording pages, are both divided into 3 bits×3 bitsareas 1-a, 2-a, . . . , 5-e, and an interval between both planes isdefined by changing an incident angle β of a reference beam over a timeT, as shown in FIG. 17, i.e., a block having two planes spaced by thetime T and a volume of 2 bits of plane dimension by 3 bits of verticaldimension by 3 bits of horizontal dimension.

[0098] More specifically, in FIG. 17, encoding is performed, forexample, in units of a three-dimensional cubic block of 2 bits by 3 bitsby 3 bits formed by an area 1-a on the plane A and an area 1-a on theplane B, both of which are recording pages.

[0099] A block is composed to satisfy a condition that it must includeone or more reference data bits “1” and one or more reference data bits“0” respectively specified at particular locations in the block. Here,one bit at the upper left corner of data in a 3×3 area on the plane A isused as the reference data “1,” and one bit at the upper left corner ofdata in a corresponding 3×3 area on the plane B is used as the referencedata “0.” The remaining 16 bit locations are assigned for informationbit data.

[0100] According to the embodiment, when the blocked data as illustratedin FIG. 17 is read, the read data is divided into 2×3×3 blocks identicalto those upon encoding at Step S22. At Steps S23, the reference data “1”recorded at the upper left corner on the plane A and the reference data“0” recorded at the upper left corner on the plane B are extracted ineach block. Then, at Step S24, the slice level is determined from theextracted reference data with which each bit of the read data isdetermined as to whether it is “1” or “0.”

[0101] Thus, the “1”/“0” determination is performed by using neighboringdata on a three-dimensional format of data to be read, i.e., thereference data within a three-dimensional block over a previously settime interval T to set a slice level. Therefore, even if the level ofread signal fluctuates due to noise caused by a variety of factors, thevalues of data within the block, which are referenced for the slicelevel, also fluctuate to cause the slice level to fluctuate in a mannersimilar to the data, thereby making it possible to correctly determinewhether the read data is “1” or “0.” The approach for setting a slicelevel using three-dimensional blocks as mentioned is advantageouslycapable of correcting fluctuations in a recording form and areproduction form on the time base, and capable of processing a largeamount of data in blocks.

[0102] As described above, even if the amplitude of two-dimensionally orthree-dimensionally arranged data spatially or temporally fluctuates dueto a variety of factors, the reference signals “1” and “0,” which aresimultaneously affected by the fluctuations are located in the vicinityof other data within the blocks, thereby making it possible to determinecorrectly whether data is “1” or “0” from the reference signals “1” and“0”.

[0103] The embodiment also provides a proper slice level and “1”/“0”determination in cases shown in FIGS. 8 and 9, in a manner similar tothe first embodiment.

[0104] While the foregoing explanation has been made referring to anexample where the shape of block for determining the slice level is 3×3for a two-dimensional block, and 2×3×3 for a three-dimensional block,any shape of block may be used as long as it includes the reference data“1” and “0” which are specified at determined locations (strictly, thelocations of which are recognizable upon decoding). Alternatively, avariety of blocks may be combined without limiting the shape. Further, ablock having a different shape may be provided for extracting thereference data “1” and “0” separately from the block for determiningwhether the data is “1” or “0”.

[0105] Also, the block may include a plurality of reference data “1” and“0,” respectively. Further, the reference data “1” and “0” may be placedat different locations from block to block.

[0106] Furthermore, while the foregoing embodiment uses binary dataconsisting of “1” and “0” as data to be recorded and reproduced,multi-value data having three values or more may also be recorded andreproduced. In this case, a plurality of slice levels may be calculatedfrom the reference data “1” and “0” to determine the multi-value data.Alternatively, other reference data indicative of any intermediate valuemay be recorded other than the reference data “1” and “0” to determinethe multi-value data.

[0107] While the respective embodiments described above cope withirregular luminance in unitary recording page for each block, a thirdembodiment described below provides a method which can cope withirregular luminance in a unitary recording page without the need fordividing the unitary recording page into blocks.

[0108] In the following, explanation is given of multiplex recordingwhich records a modulated signal beam at the same location in theholographic memory medium 1 for each page by changing the incident angleβ of the reference beam in predetermined steps. In this case, separatefrom normal recording pages for carrying normal information datapatterns, a data pattern serving as the basis of correcting forirregular luminance is recorded as a reference page. Upon reading data,the reference page is read to detect irregular luminance in thereference page based on a known pattern, and the value of informationdata in a recording page is corrected in accordance with the detectionresult.

[0109]FIG. 18 shows a relationship between a normal recording page andthe reference page.

[0110] It can be seen from FIG. 18 that the first reference page isassigned to the first page, and subsequently a reference page isassigned to every ten normal recording pages. The reference page on Page1 functions to correct the values of information data on recording pageson Page 2 to 5; a reference page on Page 11 functions to correct thevalues of information data on recording pages on pages 6 to 15; and soon. A reference page on the {10(n−1)+1}th page functions to correct thevalues of information data on normal recording pages on Pages (10n−4) to(10n+5).

[0111] The reference page on Page 1 is an initial reference page whichbelongs to a different category from the other reference pages. Thispage has the responsibility of identifying this page as the firstrecording page in the holographic memory medium 1.

[0112] Each of the reference pages is coded in the same manner as datapatterns recorded on a normal recording page, and recorded therein is arandom pattern having a known bit position relationship.

[0113] The encoder 11 inserts the reference page for correctingirregular luminance as mentioned into normal recording pages to composea data sequence to be recorded.

[0114] Each of the pages recorded in the holographic memory medium 1 isread therefrom by sequentially changing the incident angle β of thereference beam, and a read signal is supplied to the decoder 23. In thisevent, the decoder 23 corrects irregular luminance based on the readreference page. FIG. 19 is a flow chart illustrating a data reproducingprocedure including such correction.

[0115] Referring to FIG. 19, the decoder 23, upon detecting the initialreference page by identifying it (step S31), reads a normal page (stepS32), and then reads data on a corresponding reference page (step S33).

[0116] When reading data on the reference page, the decoder 23calculates a correction coefficient (step S34), and corrects data on thenormal recording page using the correction coefficient (step S35). If nodata on the reference page is read, the decoder 23 corrects data on thenormal recording page using a previously calculated correctioncoefficient corresponding thereto (step S35).

[0117] The data correction as mentioned is followed by the datareproduction processing explained in the respective embodiments inconnection with FIG. 10 or 11 (step S36). In this way, a correct slicelevel is set after data on the entire page have been corrected, with theresult that a digital signal is reliably reproduced.

[0118] After Step S36, it is determined whether or not necessary pageshave been read (step S37). If not, the procedure returns to Step S32 tocontinue to read other pages.

[0119] If necessary pages have been read, the decoder 32 performsdecoding including the processing for releasing the paged configuration(step S38), followed by the termination of the processing illustrated inthe flow chart.

[0120] Next, the data correcting method conducted at Steps S34 and S35will be described in detail.

[0121] A data pattern on a reference page have their black portions andwhite portions arranged at bit positions which have previously beenrecognized on the decoder side. Therefore, the decoder 23 can recognizewhether each of bits arranged in a two-dimensional matrix carries blackor white, in other words, which of the digit levels is assigned to.

[0122] In a white portion, the value of the read data corresponding tothe white portion itself corresponds to the amount of irregularluminance. In other words, this is comparable to detection of adeviation from an expected value with respect to the value of a readsignal corresponding to data bit which has been identified as “1.”

[0123] In a black portion, on the other hand, the amount of irregularluminance therein is estimated from irregular luminance of a whileportion adjacent to or surrounding the black portion.

[0124] It is therefore possible to detect the amount and distribution ofirregular luminance over the entire page including all of white andblack portions. For example, irregular luminance including a “dark”portion as illustrated in FIG. 3 can be detected as the amount anddistribution with respect to the entire page.

[0125] The irregular luminance thus detected may be represented in theform of two-dimensional data in units of pages. Then, thetwo-dimensional data carrying the irregular luminance is standardized, acorrection coefficient (i.e., corresponding to a coefficient forreducing the deviation to zero) is calculated and stored for each pixel,and data on a corresponding normal recording page is multiplied by thecorrection coefficient to remove the irregular luminance.

[0126] According to this embodiment, irregular luminance is detected ina page, and the value of corresponding data is corrected to remove theirregular luminance. Thus, irregular luminance between holograms causedby an uneven light source, interference fringes remaining in optics, andthe characteristics of recording materials is removed, so thatreproduced data can be reliably provided while reducing read errors.Furthermore, by combining the data reproduction processing unique to therespective embodiments described above, such as that at Step S36, theirregular luminance can be processed such that a rough compensation isfirst made and then followed by a fine compensation, thereby making itpossible to more reliably reproduce a digital signal.

[0127] While the respective embodiments described above have employedonly a holographic memory medium as a recording medium, the presentinvention is not necessarily limited to this particular recordingmedium.

[0128] While a variety of means and steps have been explained in alimited manner in the respective embodiment described above, they may beappropriately modified within a scope in which those skilled in the artmay design.

[0129] According to the present invention as described above in detail,a recorded digital signal can be reliably reproduced even if a readsignal is experiencing level fluctuations due to noise.

[0130] The present invention has been described with reference to thepreferred embodiments thereof. It should be understood that a variety ofalterations and modifications may be contemplated by those skilled inthe art. It is intended that all such variations and modifications areencompassed in the appended claims.

What is claimed is:
 1. A method of recording a plurality of digitalsignal sequences at predetermined unitary recording areas in a recordingmedium, respectively, comprising the steps of: transforming each of saiddigital signal sequences to be recorded into blocks; including aplurality of bit data carrying all digit levels to be read anddetermined upon reproduction within each of the resultant data blocks;and recording a sequence of the data blocks at the corresponding ones ofsaid predetermined unitary recording areas in said recording medium. 2.A method of reproducing digital signal sequences from a recordingmedium, comprising the steps of: recognizing blocks in each of readsignal sequences obtained from said recording medium; determining athreshold value for each block on the basis of read values of bit datacarrying all digit levels to be read and determined; and reading anddetermining each digit level of bit data of said read signal sequenceson the basis of said threshold value.
 3. A method of recording aplurality of digital signal sequences at predetermined unitary recordingareas in a recording medium, respectively, comprising the steps of:transforming each of said digital signal sequences to be recorded intoblocks; locating reference bit data carrying at least first and seconddigit levels to be read and determined upon reproduction at respectivepredetermined positions in each of the resultant data blocks; andrecording a sequence of the data blocks at the corresponding ones ofsaid predetermined unitary recording areas in said recording medium. 4.A method of reproducing digital signal sequences from a recordingmedium, comprising the steps of: recognizing blocks in each of readsignal sequences obtained from said recording medium; determining athreshold value for each of said blocks on the basis of read values ofreference bit data carrying at least first and second digit levels to beread and determined, said reference bit data being located atpredetermined positions within each of said block; and reading anddetermining each digit level of bit data of said read signal sequenceson the basis of said threshold value.
 5. A digital signal recordingmethod according to claim 1 , wherein said data block is formed of thenumber of bits corresponding to a vertical length by which a recordingplane in said recording medium is divided, and the number of bitscorresponding to a horizontal length by which the recording plane insaid recording medium is divided.
 6. A digital signal recording methodaccording to claim 1 , wherein said data block is formed of the numberof bits corresponding to a vertical length by which a recording plane insaid recording medium is divided, the number of bits corresponding to ahorizontal length by which the recording plane in said recording mediumis divided, and the number of bits corresponding to a recording timeinterval for a recording area defined by both of said numbers of bits.7. A method of recording a plurality of digital signal sequences atpredetermined unitary recording areas in a recording medium,respectively, comprising the steps of: preparing correction digitalsignals each corresponding to each of said predetermined unitaryrecording areas and each including bit data carrying digit levels to bedetermined upon reproduction and having a predetermined bit positionrelationship corresponding to said digit levels; mixing each of saidcorrection digital signals with each of said digital signal sequences;and recording the resultant digital signal sequences at thecorresponding ones of said predetermined unitary recording areas in saidrecording medium.
 8. A method of reproducing digital signal sequencesfrom a recording medium, comprising the steps of: recognizing correctiondigital signals for each of predetermined unitary recording areas fromthe read signal sequences obtained from said recording medium;identifying bit data carrying digit levels to be read and determinedfrom said correction digital signals on the basis of a predetermined bitposition relationship thereof; detecting deviation values from therespective expected values of the identified bit data; and correctingeach value of bit data of said digital signal sequences on the basis ofsaid deviation values.
 9. A digital signal recording method according toclaim 3 , wherein said data block is formed of the number of bitscorresponding to a vertical length by which a recording plane in saidrecording medium is divided, and the number of bits corresponding to ahorizontal length by which the recording plane in said recording mediumis divided.
 10. A digital signal recording method according to claim 3 ,wherein said data block is formed of the number of bits corresponding toa vertical length by which a recording plane in said recording medium isdivided, the number of bits corresponding to a horizontal length bywhich the recording plane in said recording medium is divided, and thenumber of bits corresponding to a recording time interval for arecording area defined by both of said numbers of bits.